Most routers today are Layer 3 IP routers that examine the source and destination IP addresses of each packet, look up the destination of the packet in the router's IP routing table, and route that packet on its way toward the destination. In the event that the destination is not listed in the routing table, the router will generally either send the packet to a default route (if it has one) or drop the packet. Routers are usually used to connect a local area network (LAN) to a wide area network (WAN), but can also be used to segment large LANs. During certain network failures, however, a forwarding loop can occur between a router (or other stateful device) and another device.